Spitzer Space Telescope - General Observer Proposal #40142


Resolving the cooling in the powerful H2 emitting shock in Stephan's Quintet

Principal Investigator: Philip Appleton
           Institution: California Institute of Technology

     Technical Contact: Philip Appleton, California Institute of Technology

Co-Investigators: 
Kevin Xu, NHSC-Caltech                                                
William Reach, Planck Science Center-Caltech                          
Francois Boulanger, Institut d'Astrophysique de Paris                 
Patrick Ogle, SSC-Caltech                                             
Pierre-Alain Duc, CEA-Saclay                                          
Nanyao Lu, NHSC-Caltech                                               
Richard Tuffs, MPI-Heidelberg                                         
Cristina Popescu, MPI-Heidelberg                                      
Min Yun, UMASS                                                        
Guillaume Pineau des Forets, Institut d'Astrophysique-Paris           

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IrsMap IrsPeakupImage IrsStare MipsPhot 
  Hours Approved: 32.5 

Abstract:
Recent Spitzer observations have discovered a potentially important new class of
galaxy system in which a significant fraction of the bolometric luminosity from
the system arises in the Mid-IR emission lines of molecular hydrogen. This
emission is thought to arise in large-scale shocks. We propose to study the
nearest and best-studied example of this effect which arises in a group-wide
shock wave in Stephan's Quintet. The shock-wave, which extends for over 1
arcminute throughout the intragroup medium, is believed to be the result of a
~1000 km/s collision between an infalling galaxy and the group. Previous limited
Spitzer IRS observations by our team showed extremely strong, unusually broad
emission lines of the H2 and almost nothing else. The discovery of such powerful
emission from a high-velocity shock was unexpected because of the fragile nature
of the H2 molecule, and yet numerous examples are being discovered at higher
redshift in other group and cluster environments. We propose to critically test
new models of the process which might give rise to powerful H2 emission in the
cooling layer behind a shock. By taking advantage of the large angular size of
the giant X-ray shock we can map both the H2 excitation temperature and dust
SEDs over the shocked region. Our proposed observations will help us isolate the
most important model parameters that are needed to explain the extreme
luminosities being emitted during the cooling of H2. Such models are necessary
if we are to make progress understanding these ultra-powerful H2-emitting
galaxies at higher redshift.